Polyimides are high-performance macromolecules which are generally obtained via polycondensation of aromatic and/or alicylic dianhydride and diamine structures [E. Pinel, D. Brown, C. Bas, R. Mercier, N. D. Alberola, S. Neyertz. Chemical Influence of the dianhydride and the diamine structure on a series of copolyimides studied by molecular dynamics simulations. Macromolecules. 2002; 35:10198-209].
These aromatic polyimides have been used in many high technology fields due to their excellent thermal, mechanical and electrical properties [Y. Li, X. Wang, M. Ding, J. Xu. Effects of molecular structure on the permeability and permselectivity of aromatic polyimides. J Appl Polym Sci. 1996; 61:741-8].
Among those applications, gas separation using polyimides has attracted great interest, because polyimides have significantly better permselective performance than typical glassy polymers such as cellulose acetate and polysulfone [A. Bos, I. G. M. Punt, M. Wessling, H. Strathmann. Plasticization-resistant glassy polyimide membranes for CO2/CH4 separations. Sep Purif Technol. 1998; 14:27-39].
In addition, high temperature polymers (e.g., polybenzimidazole, polybenzoxazole and polybenzothiazole) have drawn a great deal of attention due to their potential of obtaining superior gas separation performance under harsh conditions. In order to use the polymers for membrane materials, mild fabrication processes are required instead of using acidic solvents.
For example, fluorinated polybenzoxazole membranes can be synthesized by solution cyclization techniques using mild solvents [W. D. Joseph, J. C. Abed, R. Mercier, J. E. McGrath. Synthesis and characterization of fluorinated polybenzoxazoles via solution cyclization techniques. Polymer. 1994; 35:5046-50]. Their gas permeability increases according to the degree of cyclization of benzoxazole rings because increases in solubility and diffusivity coefficient are observed after cyclization [K. Okamoto, K. Tanaka, M. Muraoka, H. Kita, Y. Maruyama. Gas permeability and permselectivity of fluorinated polybenzoxazoles. J Polym Sci Pol Phys. 1992; 30:1215-21].
Meanwhile, Burns and Koros proposed a polymeric molecular sieve concept using ultrarigid polymers which exhibited entropic selectivity capabilities [R. L. Burns, W. J. Koros. Structure-property relationships for poly(pyrrolone-imide) gas separation membranes. Macromolecules. 2003; 36:2374-81]. Poly(pyrrolone-imides) composed of open regions and bottleneck selective regions can mimic molecular sieves by tuning the polymeric matrix through the use of different monomer stoichiometry.
In an attempt to find the ways to improve gas permeability, the inventors of the present invention have conducted research based upon the fact that copolymerization of high temperature polymers and polyimides results in higher gas separation performance. As a result, the present inventors have disclosed polymer structures acting as permeable sites and considered incorporating the polymer structures into polyimide backbones.
Consequently, the present inventors ascertained that aromatic polymers interconnected with heterocyclic rings (e.g., benzoxazole, benzothiazole and benzopyrrolone) showed higher gas permeation performance due to their well-controlled free volume element formation by thermal rearrangement in the glassy phase. In addition, these materials have a flat and rigid rod structure with high torsional energy barriers to rotation between respective rings. An increase in rigidity of polymer backbones with high microporosity showed positive effects in improving gas separation performance.